Add ai_web_search_invoked audit event type

What does this MR do and why?

When Duo Chat searches the web, that should leave a record for administrators. This MR registers a new ai_web_search_invoked audit event type, so web searches can be reviewed in the audit log and streamed to external destinations like any other audit event.

GitLab only stores audit events whose name it knows, so this has to be in place before the searches themselves can be recorded.

References

• Issue: #604065
• The AI Gateway side that sends the event: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!7109.

Screenshots or screen recordings

Not applicable: nothing changes in the UI beyond a new row appearing in the existing audit events list.

How to set up and validate locally

Follow testing instructions in gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!7109.

MR acceptance checklist

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